1. Field of the Invention
The present invention relates to a switch-mode power supply (SMPS), and more particularly to an SMPS with control of its output supply voltage and overcurrent which can simplify its construction and reduce its manufacturing cost.
2. Description of the Prior Art
Typically, an SMPS may be classified into a constant-voltage type SMPS and a constant-current type SMPS. In the constant-voltage type SMPS, a commercial AC supply voltage is converted into a DC supply voltage by rectification and filtering, and the DC supply voltage is converted into a rectangular waveform voltage of a high frequency by means of a semiconductor power-switching element. The rectangular waveform voltage is then applied to a primary winding of a transformer having a predetermined turn ratio, and a DC supply voltage is obtained by rectifying and filtering a waveform voltage developed in a secondary winding of the transformer. In the constant-current type SMPS, the AC supply current is converted into the DC supply current through a constant-current type power supply.
A prior art SMPS is illustrated in FIG. 1. Referring to FIG. 1, the prior art SMPS includes an output supply voltage control section 10, an overcurrent control section 20 and a driving voltage generating section 30.
Referring again to FIG. 1, the prior art SMPS also includes a transformer T1 whose primary winding is coupled to a power supply input terminal Vin, rectifying diodes D1 and D2 for rectifying the voltage developed in a secondary winding of the transformer T1, a coil L1 and a capacitor C2 for smoothing the unregulated DC supply voltage provided from the rectifying diodes D1 and D2 to provide the smoothed DC output supply voltage Vo to a load RL.
The output supply voltage control section 10 comprises resistors R1 and R2 for dividing the output supply voltage Vo, a shunt regulator SR1, whose reference voltage terminal Vref is connected to the connection point of the resistors R1 and R2, for stabilizing the output supply voltage, and a first photodiode PD for feeding an output supply voltage control signal back to the primary winding of the transformer T1 in accordance with the output voltage through a resistor R9 and the output voltage of the shunt regulator SR1. The photodiode PD constitutes a photocoupler.
The overcurrent control section 20 comprises a sensing resistor Rs for sensing the output supply current, an operational amplifier OP1 coupled between both terminals of the sensing resistor Rs through resistors R4 and R5, and a second photodiode PD2, which constitutes another photocoupler, for providing an overcurrent control signal to the primary winding of the transformer T1, being turned on or off according to the output signal of the operational amplifier OP1.
As shown in FIG. 1, the supply voltage Vcc2 is applied to the anode of the second photodiode PD2 through a resistor 8. In a normal current state, the supply voltage Vcc2 is applied to the non-inverting terminal of the operational amplifier OP1 through a resistor R6.
The driving voltage generating section 30 comprises an additional secondary winding of the transformer T1 having a predetermined number of turns, a diode D3 for rectifying the voltage induced in the additional secondary winding, and a regulator 31 for regulating the rectified voltage to provide a predetermined supply voltage Vcc2 for driving the operational amplifier OP1 in the overcurrent control section 20.
The prior art SMPS is also provided with a first phototransistor PT1 of the photocoupler which is turned on by the light generated by the photodiode PD in the output supply voltage control section 10, a second phototransistor PT2 of the other photocoupler which is turned on by the light generated by the photodiode PD2 in the overcurrent control section 20, a pulse width modulation (PWM) section 11 whose output duty cycle varies when either of the first and second phototransistors is turned on, and a switching transistor Q1 whose on/off time is controlled according to the output duty cycle of the PWM section 11 and which switches the input supply voltage Vin to the primary winding of the transformer T1.
The operation of the prior art SMPS as constructed above will now be explained.
The input supply voltage Vin is applied to the primary winding of the transformer T1 under the control of the switching transistor Q1. The voltage developed in the secondary winding is rectified by the diodes D1 and D2 and then smoothed by the coil L1 and the capacitor C1 to be provided to the load RL as the DC output supply voltage Vo.
The supply voltage Vo is divided by the resistors R1 and R2, and the divided voltage is then applied to the reference voltage terminal Vref of the shunt regulator SR1.
At this time, if the output supply voltage Vo is relatively low, the level of the reference voltage terminal Vref of the shunt regulator SR1 becomes less than 2.5 V. This causes the current flowing through the first photodiode PD of the photocoupler to become lesser.
If the output voltage of the shunt regulator SR1 falls below the reference level, the impedance of the first phototransistor PT1 increases, so that the voltage Vc supplied to the PWM section 11 increases. This results in increase of the output duty cycle of the PWM section 11. If the duty cycle of the PWM section 11 increases, the on-time of the following switching transistor Q1 also increases, resulting in increase of the output supply voltage Vo.
Meanwhile, if the output supply voltage Vo is relatively high, the level of the reference voltage terminal Vref of the shunt regulator SR1 rises above 2.5 V, and this causes the current flowing through the first photodiode PD of the photocoupler to become greater.
Accordingly, when the output voltage of the shunt regulator SR1 rises above the reference level, the impedance of the first phototransistor PT1 decreases, so that the voltage supplied to the PWM section 11 decreases. This results in decrease of the output duty cycle of the PWM section 11. If the duty cycle of the PWM section 11 decreases, the on-time of the switching transistor Q1 also decreases, resulting in decrease of the output supply voltage Vo.
The overcurrent control section 20 determines whether overcurrent flows and controls the overcurrent by comparing the current flowing though the sensing resistor Rs in the overcurrent control section 20 with a predetermined value. That is, in a normal state, the current I flowing through the sensing resistor Rs becomes less than the reference current Iref.
Accordingly, the input voltage Ve of the operational amplifier OP1 becomes higher than "0" level by the combination of the resistors R6, R5 and R4, and thus the current flowing through the photodiode PD2 of the photocoupler decreases. As a result, the impedance of the second phototransistor PT2 increases, causing the voltage being provided to the PWM section 11 to be kept unchanged.
Meanwhile, if overcurrent flows, that is, if the current I through the sensing resistor becomes larger than the reference current Iref, the input voltage Ve of the operational amplifier OP1 becomes lower than "0" level, causing the current flowing through the photodiode PD2 of the photocoupler to increase.
Accordingly, the impedance of the second phototransistor PT2 decreases, and this causes the voltage being provided to the PWM section 11 to decrease, resulting in decrease of the output duty cycle of the PWM section 11. If the duty cycle of the PWM section 11 decreases, on-time of the transistor Q1 also decreases, resulting in decrease of the output supply voltage Vo and the output supply current. Thus, overcurrent is prevented.
The voltage developed in the additional secondary winding of the transformer T1, which has a predetermined number of turns, is rectified by the diode D3 and then regulated by the regulator 31 to be provided to the operational amplifier OP1 as its driving power supply Vcc2.
However, the prior art SMPS of FIG. 1 has the drawback that it has a complicated structure since the output supply voltage control section and the overcurrent control section are separately designed. Further, its size and manufacturing cost increase since the power supply for driving the operational amplifier in the overcurrent control section should be provided by means of the driving voltage generating section including the additional winding of the transformer.